The fix for IBOC

August 1, 2003

The evolution of HD Radio is currently in a state of suspended
animation, thanks to a decision taken a few weeks ago by the National
Radio Systems Committee.

The NRSC dropped a small but significant bomb by announcing that it
was halting the standards-setting process for HD Radio because of a
problem with compression algorithms. Its DAB subgroup was worried that
the PAC compression algorithm used for AM radio was just not good
enough to be a systems standard and pointed to a recent demonstration
as evidence. At this demonstration, at National Public Radio, several
NRSC sources said they found the artifacts on the system using PAC
“unacceptable.”

Ibiquity Digital, which is attempting to forge a standard for
digital radio, was offered two possible options to address its IBOC
problem. The first was “tweaking” the PAC algorithm; the
second was reverting to another coding algorithm such as AAC or MPEG
— all of which are based on psycho-acoustic principles.

It has been well documented in the past 10 years that the number of
destructive psycho-acoustic-based algorithms in the audio broadcast
chain could, in theory, introduce artifacts. It is arguable that the
use of PAC as the final emission for IBOC simply highlights this
problem, but then again, implementation of a low bit rate algorithm for
the final leg of HD Radio was always going to accentuate existing
problems as well as introduce new artifacts. Using other
psycho-acoustic-based compression algorithms, i.e. AAC or MPEG, may
marginally alleviate the artifacts but this would only offer an
incremental improvement. One “tweaked” algorithm can only
offer a very slight incremental improvement, and even that is largely
dependent on the source material — for example, talk or music. To
fundamentally confront the problem, broadcasters need to address the
entire broadcast chain, which suggests that the current approach is not
actually addressing the true problem.

We at APT — Audio Processing Technology - have long been
recommending that broadcasters take more care of their content. We
suggest a reduction in the number of psycho-acoustic passes in the
broadcast chain and, where compression is required, the use of a
non-destructive gentler ADPCM compression algorithm such as apt-X; a
concept that has been proven with GWR and The Capital Group, following
the launch of DAB in the UK.

Until broadcasters take ownership of the problem, then the artifacts
in audio emitted via IBOC will remain. The only point of contention is
how noticeable these artifacts will be to the listener. If AM HD Radio
output appears in the form of gritty, unnatural-sounding voices then
artifacts will be noticed. Even if the artifacts are slightly subtler,
Listener Fatigue will be the final result.

Research suggests that listener fatigue makes people
“tune-out” by flicking across the dial. In the world of
commercial radio, losing listeners ultimately equates to a loss of
revenue as advertising sales rely on healthy audience figures.

To summarize in the form of an analogy, if a house has serious
cracks in its walls, then papering them over would not solve the
problem. The only way to stop the house from falling down is to take a
good look at its foundations and see what needs to be done. The same
applies to the current problems faced by IBOC. Until broadcasters take
ownership of the cascading and concatenation issue and reduce the
number MPEG passes, then all investments in HD Radio will have a very
poor return.Jon McClintock
commercial director, APT
Belfast, Ireland